As shown in FIG. 1, a muscle-building exercise device of the prior art comprises a frame 1 which is provided with a slide member 2 connected with a plurality of elastic members 3 having a damping effect. The slide member 2 can be kept at a first position which is located at the front section of the frame 1. The slide member 2 is further connected with a pull cord 4 which can be pulled to locate the slide member 2 at the second position opposite to the first position. The exercise effect is brought about by pulling repeatedly the pull cord 4 so as to cause the slide member 2 to slide back and forth on the frame 1. The magnitude of the exercise effect can be adjusted by increasing or decreasing the number of the elastic members 3.